1. Field of the Invention
The present invention relates in general to solar cells, and more particularly, to a method of applying a passivating layer to the back surface of a solar cell wafer and heavily doping selected areas in the back surface of the cell through the passivating layer.
2. Description of Related Art
Conventional solar cells consist of a semiconductor material with an n-p junction interfacing n-type and p-type impurity doped regions and a front light-receiving major surface (the emitter) and a back major surface (the bulk). When light energy impinges on the front light-receiving surface of the cell electrons and corresponding holes are created in both the emitter and bulk. For the most part, because of the presence of the n-p junction, electrons will be directed toward one major surface of the cell and holes toward the other major surface, resulting in a photocurrent density. In a typical n-p junction solar cell, electrons move to the front light receiving surface of the cell and holes toward the back surface. Electrical contacts are attached to the front and back surfaces of the solar cell semiconductor material to collect the electrons at one electrical contact and holes at the other electrical contact. The object is to collect as many electrons and holes as possible before they recombine, to attain the highest photocurrent density possible.
A portion of the electrons generated near the back major surface, however, recombine near said back surface and thus do not contribute to the photocurrent density. In order to reduce this recombination of carriers at the back surface, back surface field cells were developed in which a p+ impurity doped layer is diffused into the back surface of the solar cell. To effectively reduce back surface recombination, however, this layer must be thick. Another technique employed to reduce back surface carrier recombination involves applying a thin oxide layer to the back surface of the cell to passivate the back contact. The carriers must tunnel through this oxide layer to reach the back electrical contacts.
In U.S. Pat. No. 4,395,583, a proposed solar cell is disclosed which combines the passivated back surface cell and the back surface field cell. The back surface of the solar cell is provided with a p+ layer diffused into a p-type layer. Selected regions of the p+ layer are etched away, and an oxide layer is deposited into these selected regions, leaving the surface areas of the unetched regions of the p+ layer exposed. Metal is deposited onto the back surface making contact to these exposed p+ areas. The need for etching, however, adds considerable cost and complexity to the fabrication process. Moreover, the back surface passivation layer is limited to high quality oxide layers, such as thermally grown oxides for thin p+ layers of less than 0.3 micrometers thickness.